1. Field of the Invention
The present invention relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same, and more particularly, to a non-aqueous electrolyte for a lithium secondary battery capable of preventing the thickness of the battery from swelling when it is charged at room temperature or stored at a high temperature, and a lithium secondary battery comprising the same.
2. Description of the Related Art
The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments. Lithium secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials. The positive active materials include lithium metal oxide, and the negative materials include lithium metals, lithium-containing alloys, or materials that are capable of reversible intercalation/deintercalation of lithium ions such as crystalline or amorphous carbons, or carbon-containing composites.
Lithium secondary batteries are classified as lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries according to kinds of separator and electrolyte, and as cylindrical, prismatic, and coin-type batteries according to their shapes. A cross-sectional view of a general prismatic non-aqueous Li-ion cell is shown in FIG. 1. The Li-ion cell 3 is fabricated by inserting an electrode assembly 4 including a positive electrode 5, a negative electrode 6, and a separator 7 between the positive and negative electrodes, into a battery case 8, injecting electrolyte into the upper part of the battery case 8, and sealing the upper part of the case 8 with a cap plate 11.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7V, which is higher than those of other alkali batteries such as Ni-MH batteries, and Ni—Cd batteries. An electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate, is used as an electrolyte. However, such an electrolyte has significantly lower ion conductivity than an aqueous electrolyte that is used in a Ni-MH battery or a Ni—Cd battery, thereby resulting in the deterioration of battery characteristics during charging and discharging at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only prevents the reaction between the lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode.
Once the organic SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, such that an amount of lithium ions is maintained. That is, carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79–104). Because of these reasons, in the lithium secondary battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
However, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction (J. Power Sources, 72(1998), 66–70). These gases include H2, CO, CO2, CH4, C2H6, C3H6, etc., depending on the type of non-aqueous organic solvent and negative active material used. The thickness of the battery increases during charging due to the generation of gas inside the battery.
The passivation layer is slowly disintegrated by electrochemical energy and heat energy, which increases with the passage of time when the battery is stored at a high temperature after it is charged. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolyte occurs continuously. The internal pressure of the battery increases with this generation of gases, inducing the deformation of prismatic batteries or pouch batteries. As a result, regional differences in the cohesion among electrodes inside the electrode assembly, which is comprised of a positive electrode and a negative electrode, and a separator, occur, thereby deteriorating the performance and safety of the battery and making it difficult to mount the lithium secondary battery set into electronic equipment.
For solving the internal pressure problem, there is disclosed a method in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a current breaker or a vent for ejecting internal electrolyte solution when the internal pressure is increased above a certain level. However, the disadvantage of this method is that mis-operation of the current breaker or the fan may result from an increase in internal pressure itself.
The method in which the SEI-forming reaction is changed by injecting additives into an electrolyte so as to improve the characteristics of the battery is known. For example, Japanese Patent Laid-open No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Patent Laid-open No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved by using 1 to 20% of an N-butyl amine based compound in an electrolyte. Japanese Patent Laid-open No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−2 M of a calcium salt to the electrolyte. Japanese Patent Laid-open No. 94-333596 discloses a method in which storage characteristics of a battery are improved by adding an azo-based compound to inhibit the reaction between an electrolyte and a negative electrode of the battery. In addition, Japanese Patent Laid-open No. 95-320779 discloses a method in which CO2 is added to an electrolyte, and Japanese Patent Laid-open No. 95-320779 discloses a method in which sulfide-based compounds are added to an electrolyte in order to prevent the electrolyte from decomposing.
The above-mentioned methods induce the formation of an appropriate film such as an organic SEI firm on a negative electrode surface by adding a small amount of organic or inorganic materials to improve the storage characteristics and safety of a battery. However, the disadvantages of the above-mentioned methods are that: the added compound is decomposed or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in electrons; and gas is generated inside the battery such that there is an increase in internal pressure, resulting in significant deterioration of the storage, safety, cycle life, and capacity characteristics of the battery.